Republicans on the House of Representatives Judiciary Committee ramped up their attacks on Special Counsel Robert Mueller, who is heading the federal investigation, at a hearing.

Republicans said they had reviewed more than 300 anti-Trump text messages exchanged last year between an FBI agent and an attorney who worked on Mueller's probe.

But Rosenstein said Mueller properly removed the agent, Peter Strzok, from the probe after the texts were brought to light by the Justice Department's inspector general and added he was confident that Mueller was not letting bias seep into the investigation.

Testifying before the committee, Rosenstein said he was "not aware" of any impropriety by Mueller's team.

When asked at one point by the committee's ranking Democrat whether he had any good cause for firing Mueller, he replied: "No."

Republicans in recent months have attacked Mueller, who has so far charged four Trump associates in connection with his investigation into whether Russia meddled in the U.S. presidential election last year and colluded with Trump's campaign.

Moscow denies the allegations and Trump says there was no collusion.

Members of the committee released the contents of some of the text messages between Strzok and FBI lawyer Lisa Page.

In the texts, Trump is referred to as an "utter idiot" and "loathsome human," according to the committee's chairman Bob Goodlatte.

The texts showed "extreme bias against President Trump, a fact that would be bad enough if it weren’t for the fact that these two individuals were employed as part of the Mueller 'dream team' investigating the very person for whom they were showing disdain," Goodlatte said.

Republicans complained that many lawyers working for Mueller have donated to Democrats over the years, and could bring political bias to the investigation.

But Rosenstein said the department has strict rules that prohibit letting a person's political affiliation be considered during hiring.

"We are not going to improperly consider political affiliation with our career employees," he said.

He also said he believes Mueller is the "ideal choice" to lead the investigation, and said that just because a person is affiliated with a political party does not mean he or she will be biased.

He said he had discussed the issue of bias with Mueller and that Mueller "is running that office appropriately."

Democrats downplayed the texts and other complaints, saying there was no evidence of bias.

"Peter Strzok did not say anything about Donald Trump that the majority of Americans weren't also thinking at the same time," said the committee's top Democrat, Jerrold Nadler.